The present application claims priority to Japanese Application(s) No(s). P2000-269446 filed Sep. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
This invention relates to a light control device wherein incident light enters thereinto after control in quantity of the light and also to a pickup device using the same.
In general, a polarizer is used in a light control device using a liquid crystal cell. For the liquid crystal cell, there is used, for example, a TN (twisted nematic) liquid crystal cell or a guest-host (GH (guest-host)) liquid crystal cell.
FIGS. 14A and 14B are, respectively, a schematic view showing the working principle of a known light control device. This light control device is constituted mainly of a polarizer 1 and a GH cell 2. The GH cell 2 is sealed between two glass substrates and has working electrodes and liquid crystal alignment films although not particularly shown in the figures. The GH cell 2 has liquid crystal molecules 3 and dichromatic dye molecules 4 sealed therein.
The dichromatic dye molecules 4 have anisotropy with respect to the absorption of light and are made, for example, of positive-type (p-type) dye molecules that absorb light along the major axis of the molecules. The liquid crystal molecules 3 have dielectric anisotropy of the positive type (p-type).
FIG. 14A shows a state of the GH cell 2 in case where no voltage is applied thereto (or under conditions of applying no voltage). Incident light 5 is linearly polarized after transmission through the polarizer 1. In FIG. 14A, the polarization direction and the direction of the major axis of the dichromatic dye molecules 4 are coincident with each other, so that the light is absorbed with the dichromatic dye molecules 4, thereby causing the transmittance of the GH cell 2 to be lowered.
When a voltage is applied to the GH cell 2 as shown in FIG. 14B, the liquid crystal molecules are turned toward a direction of an electric field, under which the direction of the major axis of the dichromatic dye molecules 4 becomes perpendicular to the direction of polarization of the linearly polarized light. Thus, the incident light 5 is transmitted by the GH cell without being absorbed.
It will be noted that where negative type (n-type) dichromatic dye molecules which are capable of absorbing light along the direction of the minor axis of the molecules are used, the light is not absorbed under conditions where no voltage is applied, but is absorbed when a voltage is applied, unlike the case using the positive type dichromatic dye molecules 4.
With the light control device shown in FIGS. 14A and 14B, a ratio between the absorbances under voltage-applying conditions and no voltage-applying conditions, i.e. an optical density ratio, is at about 10. This optical density ratio is two times higher than that of a light control device constituted of the GH cell alone without use of any polarizer 1.
The optical density ratio is influenced by the gap or distance between the two glass substrates constituting the GH cell (hereinafter referred to as cell gap).
More particularly, the greater the cell gap is or the thicker the liquid crystal layer is, the greater the difference becomes between light transmittances under transparent or transmitting conditions and light-shielding conditions. Thus, although the optical density ratio (contrast ratio) can be made great, the light transmittance under transmitting conditions become lower.
When the cell gap is changed, the response speed of a light control device using the GH cell also changes. More particularly, when the cell gap is made greater, the response speed certainly tends to become slower.
Under these circumstances, there is a strong demand for a light control device using a liquid crystal cell, which ensures a great contrast ratio and, at the same time, can realize a rapid response speed.
It is accordingly an object of the invention to provide a light control device wherein while ensuring a satisfactory optical density ratio (contrast ratio) of a liquid crystal optical element, a liquid crystal optical element that can be driven at such a high response speed as required for a light control device is employed. It is another object of the invention to provide by pickup device having a light control device of the type mentioned above arranged in a light path, thereby improving properties, image quality and reliability thereof.
The invention contemplates to provide a light control device comprising a liquid crystal optical element having a liquid crystal sealed inbetween substrates (e.g. a pair of glass substrates) provided by face-to-face relation with each other wherein the liquid crystal which is made of a guest-host liquid crystal which is made of a negative type liquid crystal as a host material, and a cell gap between the substrates is at least controlled to be in the range of 2 xcexcm to 4 xcexcm in an effective light path. The invention also contemplates to provide a pickup device having the light control device arranged in a light path of a pickup system.
The light control device and the pickup device of the invention make use of a guest-host type liquid crystal element arranged in a light path thereof wherein a negative type (wherein a dielectric anisotropy (xcex94∈ is negative) liquid crystal is used as a host material. Accordingly, the light transmittance under light transmitting conditions (especially, under transparent conditions) is significantly improved by the case using a positive type (i.e. a value of xcex94∈ is positive) liquid crystal. In addition, the cell gap is at least defined within a range of more than 2 xcexcm to less than 4 xcexcm at least in an effective light path, so that a response speed can be made greater while keeping a high ratio between the optical densities (contrast ratio) under light transmitting conditions (transparent conditions) and the light intercepting condition (light-shielding conditions).
The invention provides a liquid crystal optical element having an improved optical function for use as a light control device and relies on an invention of earlier Japanese Patent Application No. Hei 11-322186, assigned to the same assignee. According to this invention, a light control device is constituted of a liquid crystal element and a polarizer arranged in a light path incident on the liquid crystal element, and a guest-host type liquid crystal using a negative type liquid crystal as a host material. Eventually, a ratio between absorbances under no voltage applying conditions and voltage applying conditions (i.e. a ratio between optical densities) is improved by a great contrast ratio of the light control device. Thus, one enables to normally conduct light control operations in bright to dark places.
In the guest-host type liquid cell (GH cell) 2 shown in FIGS. 14A and 14B, a positive type liquid crystal where a dielectric anisotropy (xcex94∈ is positive is used as a host material, a positive type dye 4 which has dichromaticity and in which a light absorption anisotropy xcex94A is positive, is used as a guest material 4, and the polarizer 1 is provided at an incident side of the GH cell 2. When a working voltage using a rectangular wave as a drive wave is applied to so as to measure a change of light transmittance, it has been found that, as shown in FIG. 15, as the working voltage increases, an average light transmittance of visible light in the air (wherein a transmittance through the liquid crystal cell along with the polarizer is taken as a reference (100%) herein and whenever it appears hereinafter) increases. Nevertheless, when the voltage increases up to 10V, a maximum transmittance arrives only at about 60%, with the gentle change of the light transmittance.
This is considered for the reason that where a positive type host material is used, liquid crystal molecules whose director does not change (or undergoes little change) in the direction, remain owing to the strong interaction of the liquid crystal molecules at the interface between the liquid crystal cell and the liquid crystal alignment film under conditions where no voltage is applied to.
In contrast, in the earlier invention, a guest-host type liquid crystal cell (GH cell) 12 is provided as shown in FIGS. 3A to 3C. In the cell 12, MLC-6608, made by Merck and Co., Inc., which is a liquid crystal of a negative type and whose dielectric anisotropy (xcex94∈) is negative, is used as a host material 13, for example, and D5 of BDH Co., Ltd., which is a positive type dye having dichromaticity, is used, for example, as a guest material 4. In this arrangement, a polarizer 11 is provided at an incident side of the GH cell 12 to measure a change in light transmittance when a working voltage is applied thereto. As a result, it has been found that, as shown in FIG. 4, when the working voltage increases, the average light transmittance (in the air) of visible light is reduced by about 75% up to several % of a maximum transmittance, with the change of the light transmittance being relatively sharp.
This is considered for the reason that where a negative type host material is used under which the interaction of liquid crystal molecules at the interface of the liquid crystal cell and the light crystal alignment film is very weak under conditions of applying no voltage, light is likely to transmit under conditions where no voltage is applied thereto and the direction of directors of the liquid crystal molecules is likely to undergo a change along with a voltage being applied to.
In the practice of this invention, the GH cell is constituted by use of a negative type host material in such a way as set out hereinabove, thereby improving a light transmittance (especially, under transparent conditions), thus enabling one to realize a compact light control device ensuring the use of the GH cell fixed into a position of a pickup optical system. In this case, when a polarizer is arranged in a light path of incident light on the liquid crystal element, a ratio between absorbances under no voltage-applying conditions and under voltage-applying conditions (i.e. a ratio between optical densities) is further improved, and the contrast ratio of the light control device becomes so great that light control operations can be normally performed in bright to dark places.
We have made intensive studies on a further improvement in characteristics of a light control device using such a cell as GH cell as described hereinabove. As a result, it has been found that as shown in FIG. 1, a ratio between optical densities under transparent and light-shielding conditions of a liquid crystal element is greatly influenced by a distance or gap between two glass substrates (i.e. a cell gap) constituting the GH cell.
More particularly, a greater cell gap, or a greater thickness of the liquid crystal layer, results in a greater difference in light transmittance between transparent and light-shielding conditions. Although a optical density ratio is taken great, the light transmittance under transparent conditions, which is one of the merits in the use of a negative type liquid crystal as a host material, becomes lower.
As is particularly shown in FIG. 2, it has also been found that the change of the cell gap results in a great change in response speed of the light control device using the GH cell, and that when the cell gap becomes great and the liquid crystal layer becomes thick, the response speed certainly becomes slower.
In view of these, it has been found that in case where a light control device using a guest-host type liquid crystal is made, there is a certain range of the cell gap, within which several characteristics that are difficult to stand together, e.g. a light transmittance under transparent conditions, a light transmittance under light-shielding conditions, and a response time of a liquid crystal element, can be satisfied or well-balanced.
More particularly, in order to realize a light control device which makes use of a liquid crystal element and is convenient for practical applications, it is necessary to keep a satisfactorily high optical density ratio without lowering a response speed, for which it has been found that the gap between the glass substrates of the GH cell (i.e. a cell gap), between which a liquid crystal is sealed, has to be set within a range of more than 2 xcexcm up to less than 4 xcexcm. The invention has been accomplished based on this finding.
In other words, when the cell gap is smaller than 2 xcexcm, the response speed for use as a light control device becomes greater and the light transmittance under transparent conditions is improved, Nevertheless, the light transmittance under light-shielding conditions inconveniently increases as well. Eventually, a high optical density ratio (contrast ratio) cannot be attained. In contrast, when the cell gap exceeds 4 xcexcm, a high optical density is ensured, but the light transmittance under transparent conditions gets lower and a response speed as a light control device is significantly degraded. Especially, when the device is so driven as to slightly change the light transmittance in a half-tone range, the response speed considerably becomes lower (Drive a in FIG. 2). In FIG. 2, where a voltage is so changed as to develop a halftone or is changed, for example from 2V to 3V as in Drive (a) in comparison with the cases of Drives (b), (c) and (d) wherein the drive is changed from transparent conditions to light-shielding conditions, e.g. from 0V to 5 V, the response speed becomes slower owing to the shortage of the voltage, with a tendency to suffer from a great influence given by the size of the cell gap.
In the practice of the invention, it is preferred, as shown in view of FIGS. 1 and 2, that the cell gap is in the range of 2 to 3.5 xcexcm, and more preferably from 2 to about 3 xcexcm.
It is also preferred that the gap at the intermediate portion of the liquid crystal cell corresponding to the effective light path is smaller than those in the vicinity of the intermediate portion. More particularly, while controlling the gap, for example, between opposing glass substrates of the GH cell constituting a liquid crystal optical element at 2 to 4 xcexcm, a gap length at the cell intermediate portion (or at the center of the cell) is made smaller than a gap length around the peripheral portion of the cell.
According to extensive studies made by us, it has been found that the dependence of a response speed (especially, a response speed under half-tone driving conditions) on the cell gap tends to be greater than that of an optical density ratio. In accordance with the invention, while keeping an optical density ratio necessary for a light control device, the liquid crystal element in the effective light path can be able to work at a higher speed.
For the formation of such a cell gas as defined hereinabove, a spacer is provided between a transparent electrode and a facing transparent substrate formed by an alignment film, and the liquid crystal cell is sealed with a sealing material along the periphery thereof. It is preferred that the sealing material is formed in a diameter larger than the spacer, or the sealing material contains a hard material in the form of balls or fibers.
It will be noted that in the light control device and pickup device of the invention, the negative type liquid crystal in the liquid crystal optical element is negative with respect to the dielectric anisotropy, and the guest material may be made of a positive or negative type dichromatic dye.
In the practice of the invention, usable negative type host materials whose dielectric anisotropy (xcex94∈) is negative, include those indicated below. It is to be noted that in practical applications, a composition obtained by blending compounds selected from those compounds indicated below may be used so as to show nematic properties within a practically employed temperature range.
Exemplified Compounds
Other Fundamental Skeletons
In the following formulas, E, R1, R2 and L, respectively, represent a linear or branched alkyl group, alkoxy group, alkenyl group, fluoroalkoxy group or fluoroalkenyl group, xe2x80x94CN or the like. 
These compounds may be commercially available under the designations indicated below, with physical properties thereof shown below.
MLC-6608 (Made by Merck and Co., Inc.)
MLC-2039 (Made by Merck and Co., Inc.)
MLC-2038 (Made by Merck and Co., Inc.)
MLC-2037 (Made by Merck and Co., Inc.)
The dichromatic dye molecules usable in the light control device based on the invention are those indicated below.
As shown, for example, in FIG. 7, a light control device 23 consisting of such a cell as GH cell 12 as set out hereinbefore is arranged between front group lenses 15 and rear group lenses 16, each constituted of a plurality of lenses like such as zoom lens. The light transmitted through the front group lenses 15 is linearly polarized via a polarizer 11 and enters into the GH cell 12. The light transmitted through the GH cell 12 is converged by means of the rear group lenses 16 and projected as a picture on an imaging surface 17.
The polarizer 11 of the light control device 23 is so designed as to be taken in or out relative to the effective light path of incident light on the GH cell 11, like in the case of the afore-mentioned earlier application, assigned to the same applicant. More particularly, when moved to the position indicated by an imaginary line, the polarizer can be taken out from the effective light path. By means of taking the polarizer 11 in and out, there may be used such a mechanical iris as is particularly shown in FIG. 8.
This mechanical iris is a mechanical iris device which is usually employed for digital still cameras and video cameras, and is composed mainly of two iris blades 18, 19, and a polarizer 11 attached to the iris blade 18. The iris blades 18, 19 can be vertically moved, respectively. The iris blades 18, 19 are relatively moved in the directions indicated by arrows 21 by use of a drive motor not shown.
In this way, as shown in FIG. 8, the iris blades 18, 19 are partially superposed. When the degree of the superposition becomes great, an opening 22 above the effective light path located in the vicinity of the center of the iris blades 18, 19 is covered with the polarizer 11.
FIGS. 9A to 9C are, respectively, an enlarged view of part of the mechanical iris in the neighborhood of the effective light path 20. Simultaneously with the downward movement of the iris blade 18, the iris blade 19 moves upwardly. This results in, as shown in FIG. 9A, the movement of polarizer 11 attached to the iris blade 18 outside the effective light path 20. In contrast, when the iris blade 18 is moved upwardly or the iris blade 19 is moved downwardly, the iris blades 18, 19 are superposed with each other. Accordingly, the polarizer 11 moves on the effective light path as shown in FIG. 9B so that the opening 22 is gradually covered. When the degree of the mutual superposition of the iris blades 18, 19 becomes great, the polarizer 11 fully covers the opening 20 therewith as is particularly shown in FIG. 9C.
Next, the light control operation of the light control device 23 using the mechanical iris is illustrated.
As a subject, not shown, comes brighter, the iris blades 18, 19 that are made open in vertical directions as shown in FIG. 9A are driven with a motor not shown, and start to be superposed. This permits the polarizer 11 attached to the iris blade 18 to gradually enter into the effective light path 20, thereby covering part of the opening 22 therewith (see FIG. 9B).
At this stage, the GH cell 12 is in a state where no light is absorbed (although a slight degree of absorption with the GH cell 12 takes place such as by thermal fluctuation or surface reflection). Hence, the light passed through the polarizer 11 and the light passed through the opening 22 becomes substantially equal to each other with respect to the intensity distribution.
Thereafter, the polarizer 11 is in the state of fully covering the opening 22 (see FIG. 9C). Further, if the subject increases in brightness, a voltage to the GH cell 12 is increased so that light is absorbed in the GH cell to control the light intensity.
In contrast, where the subject becomes dark, a voltage applied to the GH cell 12 is reduced or no voltage is applied, thereby causing the absorption of light with the GH cell not to take place. When the subject becomes further darker, a motor, not shown, is driven to move the iris blade 18 downwardly or the iris blade 18 upwardly. In this manner, the polarizer 11 is moved outside the effective light path 20 (see FIG. 9A).
As is shown in FIGS. 7, 8 and 9A to 9C, since the polarizer (having a transmittance, for example, of 40% to 50%) is moved outside the effective light path 20, so that the light is not absorbed with the polarizer. Accordingly, the maximum transmittance of the light control device can be increased, for example, to a level two times or much higher. More particularly, this light control device has a maximum transmittance, for example, of about two times that of a conventional light control device including fixedly set polarizer and GH cell. It will be noted that minimum transmittances of both types of devices are equal to each other.
The polarizer 11 is taken out or in by use of the mechanical iris that has been put into practice in the field of digital still cameras and the like, and thus, the light control device can be readily realized. Since the GH cell 12 is used, the GH cell is able to absorb light in addition to the light control with the polarizer, thus contributing to the control of light.
In this way, the light control device of the invention ensures a high contrast ratio between brightness and darkness and can also keep substantially a uniform distribution of light quantity.